The present invention pertains to a technical field of a seat weight measuring apparatus for measuring the weight of a seat for a vehicle (such as an automobile) including a load applied to the seat and, particularly, to a technical field of a seat weight measuring apparatus provided with a sensor portion comprising measuring gauges such as strain gauges for measuring force due to the seat weight.
As a seat weight measuring apparatus of this kind, there has been conventionally proposed a seat weight measuring apparatus comprising an arm to which force due to the weight of a seat for a vehicle (such as an automobile) including a load applied to the seat is applied and which transmits the force to a sensor portion (for example, see U.S. Pat. No. 6,571,647 (incorporated by reference herein)).
As shown in FIGS. 4(A) through 4(D), a seat weight measuring apparatus 100 disclosed in U.S. Pat. No. 6,571,647 has a pin bracket 101 to which the seat weight is applied, an arm 102 which transmits force due to the seat weight, a sensor portion 103 which detects the force applied from two branched tips 102A of the arm 102 to measure the seat weight, and a base frame 107.
The pin bracket 101 and the arm 102 are pivotally connected by a stopper bolt 105 extending through holes 106 formed in left and right side walls 101L, 101R of the pin bracket 101 and holes 109 formed in left and right side walls 102L, 102R of the arm 102. The stopper bolt 105 further extends through elongate holes 108 which are long in a vertical direction and are formed in left and right side walls 107L, 107R of the base frame 107.
The arm 102 is pivotally supported to the left and right walls 107L, 107R of the base frame 107 by a pivot bolt 110. The tips 102A of the arm 102 are adapted to transmit the force to upper and lower half arms 119, 120 which are fixed to force transmission portions 104C, 104D on both sides of the sensor plate 104. Therefore, the seat weight is applied to the arm 102 from the pin bracket 101 as shown in FIG. 4(B) and the force corresponding to the seat weight is transmitted to the sensor plate 104 of the sensor portion 103 via the upper and lower half arms 119, 120.
The sensor plate 104 of the sensor portion 103 is supported by a column 111 standing on the middle of the bottom of the base frame 107. The sensor plate 104 is fixed by screwing a nut 112 onto the column 111. The sensor plate 104 is elastically distorted by the force applied from the arm 102 according to the force. As shown in FIG. 5, the distortion of the sensor plate 104 is detected by four strain gauges 113, 114, 115, and 116 attached to the sensor plate 104, whereby the sensor portion 103 measures the seat weight. The four strain gauges 113, 114, 115, 116 cooperate to form a bridge circuit.
In the seat weight measuring apparatus 100, as shown in FIG. 5, the sensor plate 104 is formed to have two necks 104A, 104B. The four strain gauges 113, 114, 115, 116 are arranged in alignment such that pairs are placed symmetrically about the column 111. The reason why the sensor plate 104 is provided with the necks 104A, 104B and the four strain gauges 113, 114, 115, 116 is as follows. As force due to the seat weight is transmitted to the force transmission portions 104C, 104D of the sensor plate 104, one of the strain gauges 113, 114 in pair on one side and one of the strain gauges 115, 116 in pair on the other side are subjected to tension, while the other one of the strain gauges 113, 114 in pair on the one side and the other one of the strain gauges 115, 116 in pair on the other side are subjected to compression. By designing the sensor portion so that one of two strain gauges on the same side is subjected to tension and the other one is subjected to compression, the sensitivity of measurement by the four strain gauges arranged to form a bridge circuit is improved, thereby accurately measuring the force (i.e., the seat weight).
In case the force transmitted to the sensor plate 104 is measured by the four strain gauges 113, 114, 115, 116 which are arranged to form a bridge circuit, heat of ambient temperature of the seat weight measuring apparatus 100 is transmitted from the center of the sensor plate 104 toward the force transmission portions 104C, 104D on both sides via the bottom plate of the base frame 107 and the column 111 as shown by arrows in FIG. 5. Therefore, as the ambient temperature changes, the thermal change is also transmitted from the center of the sensor plate 104 toward the force transmission portions 104C, 104D, thus causing a temperature difference between the resistors of the inner (center-side) strain gauges 114, 115 and the resistors of the outer (force transmission portion-side) strain gauges 113, 116. Accordingly, due to a change in ambient temperature, the accuracy in measurement of force by the respective strain gauges 113, 114, 115, 116 is deteriorated.
As a result, this conventional device includes a temperature sensing resistor 117 disposed on the sensor plate 104 to compensate for the temperature differences between the resistors of the inner strain gauges 114, 115 and the resistors of the outer strain gauges 113, 116 as shown in FIG. 5.
As the ambient temperature of the seat weight measuring apparatus 100 rapidly changes, however, a temperature difference that is more than that can be compensated by the temperature sensing resistor 117 occurs between the resistors of the strain gauges 113, 114, 115, 116. That is, there is a problem that the accuracy in measurement of force by the strain gauges 113, 114, 115, 116 is deteriorated even with the temperature sensing resistor 117 when the ambient temperature is rapidly changed. As a means of solving this problem, heat insulation materials may be attached to the peripheries of the seat weight measuring apparatus. However, the heat insulation materials can not effectively inhibit the thermal change transmitted through the column 111.